import threading
import time
def print_num():
    for i in range(1,5):
        print(i)
        time.sleep(1)
def print_letter():
    a = ['a','b','c','d']
    for i in a:
        print(i)
        time.sleep(1)
# print_num()
# print_letter()
# 定义两个线程
t1 = threading.Thread(target=print_num)
t2 = threading.Thread(target=print_letter)
# 开启两个线程
t1.start()
t2.start()
# 等待线程结束 线程的同步
t1.join()
t2.join()
print("abc")

